This invention concerns ammonium nitrate prill as used in explosive formulations such as ANFO (which is ammonium nitrate (AN) prill containing absorbed fuel oil (OF) at a level of about 6% by weight or higher) and HANFO, or "Heavy ANFO", which is a blend of AN prill (or ANFO) and emulsion explosive.
Explosives grade ammonium nitrate prill (herein "EGAN" prill) is required to be sufficiently porous to be able to absorb the quantity of fuel oil needed for an effective explosives composition. Porosity, which is indicated comparatively by particle density and bulk density, is affected by the conditions of prill formation (especially the initial water content of the spray melt) and the conditions under which the residual small percentage of water (up to 5% by weight) in the collected product at the base of the prill tower is removed by a drying process (down to e.g. 0.1 to 0.2% by weight). The size of EGAN prill is usually around 2 to 3 mm diameter, but larger diameters are not excluded.
In the specification of co-pending UK Patent Application No. 91 24 304.8 a process is described for producing predominantly monosized prills of AN in a prilling tower by a modification of the process described in published EP 0 320 153. The disclosure in UK 91 24 304.8 is as follows:
"It is known from EP-A-0 320 153 that the disintegration of a liquid jet comprising a highly concentrated solution of ammonium nitrate can be controlled so as to produce a substantially monosize distribution of droplets and hence, on solidification, particles. The disintegration is controlled by causing the plate, wherein an orifice which produces the liquid jet is located, to vibrate at a predetermined frequency and in a plane substantially perpendicular to the direction of flow of the liquid jet, thereby inducing a so-called asymmetric disturbance on the surface of the liquid jet. The vibration is required to have an amplitude which is substantially smaller than that used in other systems wherein the orifice containing plate is vibrated in a plane parallel to the direction of flow of the liquid jet and/or wherein the orifice containing plate is swung from side to side. The predetermined frequency is required to be calculated according to the expression EQU f.sub.OPT =u.sub.j .multidot.(4.5d.sub.j).sup.-1 ( 1)
wherein f.sub.OPT is the frequency in Hertz, u.sub.j is the velocity of the jet from the orifice (m.s.sup.-1) and d.sub.j is the diameter of the orifice (m). The effect of applying a vibration having the predetermined frequency was confirmed using water, which flow properties are similar to those of a highly concentrated solution of ammonium nitrate, and was successfully applied to the prilling of fertilizer grade ammonium nitrate. It is known in the art that fertilizer grade ammonium nitrate is required to have a high bulk density, and to be relatively non-porous. In order to achieve these desired properties the fertilizer grade ammonium nitrate is produced from a highly concentrated ammonium nitrate solution having an ammonium nitrate concentration of at least 99% by weight, which is known in the art as ammonium nitrate melt or molten ammonium nitrate. Prilling of ammonium nitrate solutions having ammonium nitrate concentrations of less than 99% by weight, e.g. 96% by weight produce a lower density, porous, and physically weaker material which is not as suitable for use as a fertilizer.
It is also known to use ammonium nitrate as an explosive, however, the required characteristics of explosive grade ammonium nitrate are different from those of fertilizer grade ammonium nitrate in that explosive grade ammonium nitrate is required to be relatively porous so that it is able to more readily absorb the various additives and conditioners used to sensitize it to detonation. Thus, in the preparation of explosive grade ammonium nitrate it is required that a solution having a low concentration of ammonium nitrate, i.e. less than 99% w/w, is prilled and the resulting droplets solidified in a generally similar process to that used for the preparation of fertilizer grade ammonium nitrate. In the preparation of explosive grade ammonium nitrate it is also desirable to obtain a monosize distribution of particles. Application of asymmetric vibrations to the orifice containing plate at a predetermined frequency calculation according to the method disclosed in EP-A-0 320 153, however, does not lead to the preparation of explosive grade ammonium nitrate comprising particles having a monosize distribution.